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2 AN EARLY HISTORY Originated in Ancient Egypt (3000 year s ago)- monitoring of Nile River water level for flood prediction- approximately 20 recording stations (‘Nilometers’) were located - ‘Nilometer’ attached to temple walls, quays and the inside walls of temple annexes- units of measurement were ‘cubits’ (the measurement from the elbow to the tip of the middle finger, approximately 450mm to 530mm)- modern day methods of measurement have evolved from this primitive method of water level measurement

6 Manual MeasurementThe following example shows how depth of flow can be calculated by this method in a sewer.ZERODIPDEPTHFrom a fixed point a measurement is taken to water level (ie the ‘dip’)This reading is subtracted from a known measurement (ie the ‘Zero’)Depth = ‘Zero’ – ‘Dip’6

15 Gas Purge (Open) System ‘Bubbler’Basic Operating Principledry nitrogen (or air) bubbled into the stream via a small diameter tubebubbles escape from the end of the tube (orifice)pressure transducer monitors changes in pressure within the tubingavailable as ‘single’ or ‘dual’ orifice/riverline optionsSystem pressure is proportional to water level15

27 Ultrasonic Systems Ultrasonic pulses emitted by the transducerPulses are reflected by the water surface and reflected back to the transducerTime from emission to receipt of the signals is proportional to the level in the vesselMounted vertically above stream27

28 Doppler SystemsIf the distance between the transducer and the reflecting object is decreasing, frequency increasesIf the distance between the transducer and the reflecting object is increasing, frequency decreasesWater level measured by ‘vertical’ acoustic beamSystems also capable of flow measurement28

29 Radar SystemsExtremely short microwave impulses are emitted by the antennae system to the water surfaceThese impulses are reflected by the water surface and received again by the antenna systemTime from emission to reception of the signals is proportional to water level in the stream29

30 Laser SystemsChanges in water level based characterised by intensities of angular reflectance of light30